This invention relates to a fuel assembly for fast spectrum reactor and more particularly to a double articulated fuel assembly. By articulated it is meant that the fuel assemblies are provided with either joints or localized areas of increased or complete flexibility.
A typical fuel assembly for a fast spectrum or breeder reactor is of the closed housing type and requires that the housing be flexible through the high heat and neutron flux region of the core. There must be a clamping arrangement for the fuel assembly to retain the assembly in a "tight" core arrangement (orientation and position relative to neighboring assembly) for proper reactor control yet each assembly must be able to distort as hereinafter described. The upper and lower ends of each assembly are held essentially vertically and in line by the engagement of the lower end of the assembly in support plates or tube sheets and the top of the element being tightly clamped in a packed matrix of interfitting hexagonally shaped members.
The nuclear reactor fuel assemblies of a fast breeder reactor undergo physical changes along their axial length when irradiated by a non-uniform neutron flux. These changes include bowing due to differential effects of irradiation swelling and dilation (swelling) of the structural components of the housing between the two fixed ends of the element. Similarly, fuel assembly distortions occur when each assembly is subjected to adverse thermal conditions where one of its longitudinal sides is at a greater temperature than its opposite side. The fuel assembly will become distorted and tend to bow convexly in the direction of the greatest temperature. This bowing effect, whether thermally activated or due to swelling, imparts strain to the assembly housing and associated hardware particularly in the region just above the bottom support plate or tube sheet rigidly retaining the base portion of each of the fuel assemblies within the reactor core.
Both of these effects tend to cause the fuel assembly to become bent along its length between the upper and lower points of restraint. If the fuel assemblies are not allowed to undergo the physical distortions mentioned hereinabove, then this bending can lead to excessive housing stresses and/or difficulties in refueling the reactor and/or unsafe changes in the reactivity of the core.
One solution for alleviating this problem of physical distortion has been to provide a single articulating ball joint intermediate the lower end of the fuel assembly housing and the lower restraint. Such a system is described in U.S. Pat. No. 3,671,394 to Bernath et al entitled "Articulated Fuel Element Housing". The use of this ball joint permits the lower end of the fuel assembly housing to pivot or rotate about the ball joint as the housing is subjected to the neutron flux and thermal gradients in the core and distorted along its length. While such a fuel assembly has alleviated some of the problems experienced by completely rigid fuel assemblies, its use is still limited by the flexibility of the housing along its length.